Light propagates in straight line paths known as rays. Rays may be refracted, reflected, and scattered at material interfaces. In a dielectric waveguide, a high index core region may be surrounded by a lower index cladding layer. Rays may be confined in the high index core region by internal reflection at the core/cladding interface. The reflected rays may interfere with each other to form electromagnetic field patterns within the waveguide.
In a waveguide, light may have only certain allowed states called “modes.” “Modes” of a waveguide refer to field patterns that propagate in the core region without dispersion, i.e., changing shape. The waveguide could be “single mode” if it only supports one mode. “Multimode” waveguides support many modes. An analogy of a mode may be thought of as a probability function, where the mode is similar to electron shells in atoms. An electron is confined in a shell, just as a photon is found in its mode. By changing the shape of the waveguide, the waveguide mode can no longer be supported, and light may be expelled, directed, steered or “forced” out of the waveguide and into the higher index detector.
A waveguide may guide light to a photodetector. To electrically detect light in a waveguide, the photodetector may absorb radiation, collect photogenerated charge and produce an electric current. To improve detection efficiency, the absorbing detector material is typically placed in contact with the waveguide in a configuration known as “evanescent coupling.” A detector coupling efficiency may be improved by including an intermediate “matching” layer between the waveguide core and the absorbing detector material. This configuration may still suffer from scattering loss from the detector material and inefficient coupling due to mode repulsion and guiding in the matching layer.
Performance limits of a phototransistor device are defined by a gain bandwidth product. A phototransistor with high gain may have diminished frequency response or bandwidth. A “fast” phototransistor may have reduced gain or signal. The speed of a phototransistor may be limited by a transit time of the slowest carriers across the detector's active region. A photodetector may be limited by a transit time of the slowest carriers across the detector's active region.